The present invention is directed towards transducers. These devices are used to convert the physical energy of a vibrating ferromagnetic object into an electrical signal. The pickup of an electric guitar is a transducer that converts the kinetic energy of a vibrating guitar string into an electrical signal in the form of an oscillating voltage. Generally, guitar pickup transducers utilize permanent magnets and electrical coils that are formed by winding insulated copper wire around pole pieces. The transducer""s magnet and coil winding system are mounted on the body of a guitar so that the guitar strings pass through the magnet""s flux field and alter the shape of the magnetic field when the string vibrates. The changing flux induces an electrical signal in the windings of the pickup. The guitar amplifier converts this voltage into sound.
The traditional guitar has a plurality of guitar strings that are secured at each end and held under tension to vibrate at the appropriate frequency. The guitar strings are supported on a bridge over a transducer. On electric guitars with magnetic pickups, the guitar strings normally do not touch the pickup/transducer, but instead lie in close proximity thereto. This is also the case for tone-hole pickups used in acoustic guitars. The transducer includes a magnet that emits a magnetic field and an electrical coil that is placed within the effects of the magnet field. The strings are constructed from magnetically permeable material and are placed so that they pass through the transducer""s magnetic field. When plucked or strummed, the magnetically permeable material of the vibrating guitar strings produce a corresponding oscillating magnetic flux at the windings of the coil. Thus, through magnetic induction, the vibration of the guitar strings moving within the lines of magnetic flux emanating from the pickup causes an electrical signal to be generated within the coil of the pickup.
Often, during music performance or recording, the pickup signal is processed to create a desired effect. Among the most common effects are added harmonic distortion, chorus and reverberation. For some of the more sophisticated effects, such as polyphonic fuzz, it is preferred, and sometimes required, that a separate signal be obtained from each string. For this purpose, polyphonic pickups are used. A polyphonic pickup contains multiple sensors, each one being particularly sensitive to the vibrations of one string and relatively insensitive to the vibrations of other strings. A polyphonic pickup for a six-string guitar has six sensors, and is sometimes referred to as a hexaphonic pickup or a hex pickup. Polyphonic or hexaphonic guitar pickups are also used in systems where the guitar is interfaced with a digital signal processor or synthesizer where the final sound is created.
In a hexaphonic pickup, each sensor is dedicated to a different string of a six-string guitar. The two common types of pickups used for this purpose are piezoelectric and magnetic pickups. The magnetic pickup generally consists of variable reluctance type magnetic field sensors with permanent magnets and sensor coils located under the strings. This type of pickup produces output voltages in its coils in response to the velocity of the vibration of the parts of the strings that are in its magnetic field.
Variable reluctance type transducers are often used to measure or detect the velocity of a moving ferromagnetic target. When the target can move only along a predetermined path, the direction of velocity can be determined from the polarity of the voltage induced at the sensing coil of the transducer. However, if the target can move along an arbitrary path, as in the case of a section of a vibrating guitar string, the direction of movement cannot be determined from the induced voltage polarity, nor does the magnitude of the induced voltage accurately represent the magnitude of the target""s velocity.
As previously noted, polyphonic guitar pickups are often used in combination with signal processors that are designed to create different sounds depending on certain characteristics of string vibrations. This gives the guitar player a degree of expression not possible with signals obtained from monophonic pickups. Sometimes the sound may be digitally synthesized or modified using information obtained from the pickup signal. In such systems, inadequate or inaccurate conversion of string vibrations into pickup signals result in poor digital pitch tracking and unwanted sounds. It is therefore desirable for a polyphonic pickup to produce signals that are as accurate a representation of all aspects of the vibrating string as possible. Signal components caused by other sources, such as vibrations of adjacent strings, vibrations of other parts of the guitar, noises created by inadvertent impacts on the guitar body, fret noise, etc., are to be avoided as much as possible. Generally, piezoelectric pickups are more sensitive to such extraneous unwanted effects than magnetic pickups are. On the other hand, magnetic polyphonic pickups may suffer from magnetic cross talk between the strings. Cross talk can occur when a each transducer senses the vibration of adjacent strings in addition to the one immediately overlying the transducer in question. This may be caused by the second string""s vibration affecting the magnet field at the coil of the first transducer, and may also be caused by stray magnetic flux of the second transducer affecting the readings of the first transducer""s coil
When a guitar string is plucked and released, a given point on the string vibrates in multiple directions in the transverse plane. The transverse plane is the plane perpendicular to the axis of the string. The path of string vibration may be, for example, a precessing ellipse in the transverse plane. Conventional magnetic polyphonic guitar pickups respond primarily to string vibrations occurring along the vertical axis, i.e., towards and away from the pickup. They also respond, but with less sensitivity, to string vibrations occurring along the horizontal or axis, i.e., in the plane defined by the strings. As a result of this cross-axis sensitivity, string vibrations in different directions induce differently scaled voltages in the sensing coil that are inseparably mixed in the output signal. This drawback of conventional magnetic pickups limits the tracking speed, pitch accuracy, and other performance characteristics of the electronic systems that interpret the signal. As a demonstrative example, string vibrations with large amplitude in a near-horizontal direction may be indistinguishable from those with small amplitude in a near-vertical direction. Conversely, the pickup may respond with different sensitivities to string vibrations of equal amplitudes in different directions.
The insufficiency of conventional guitar pickups to determine transverse string vibration in all planes has been recognized by other inventors in the prior art. An example of a multiple pole pickup for a single string is shown in U.S. Pat. No. 4,348,930 issued to Chobanian et al. on Sep. 14, 1982 entitled Transducer For Sensing String Vibrational Movement in Two Mutually Perpendicular Planes. This patent teaches separate dedicated pole pieces and coils that are sensitive to vibration in two separate and mutually perpendicular planes. This patent is directed towards the use of a first magnetically permeable pole piece with a first coil for supplying a first electrical signal and a second magnetically permeable pole piece with a second coil for supplying a second electrical signal. The design uses a first pole piece where the vibrational movement of the string in a first plane induces minimal or insignificant flux changes in the second coil, and vice versa. Thus, the vibrational movement of the string in one plane is sensed independently of, and with minimal influence over, the sensing of the vibrational movement of the string in the other mutually perpendicular plane. Thus, Chobanian describes a polyphonic magnetic guitar pickup with two sensor coils per string having their sensitive axes perpendicular to one another. It is claimed that when the string vibrates in the sensitive plane of one of the sensors, significantly greater changes result in the magnetic flux in one pole piece than in the other pole piece. However, this device does not permit resolving the direction of string vibration onto orthogonal axes, because the magnetic fields of both sensors interfere with each other at the string and at both pole pieces. Thus, the vibration of the string in any direction results in a non-negligible voltage being induced simultaneously in both coils.
With U.S. Pat. No. 4,534,258, entitled Transducer Assembly Responsive to String Movement in Intersecting Planes, Norman J. Anderson describes a magnetic pickup designed to determine all the transverse movement of the string. In this design, too, each coil is maximally sensitive to vibration of the string in a first plane and minimally sensitive to vibration of the string in a second plane that intersects the first plane. Anderson explains that these principal planes are preferably perpendicular and at xe2x88x9245 degree and +45 degree angles with respect to the top surface of the guitar body. The signals induced by the vibrations of all strings in one set of coils are combined into one audio channel, and signals induced by the vibration of all strings in the other set of coils are combined into the second audio channel. Thus, while the vibration planes are partly distinguished, the string signals are mixed. In addition, with the described device vibration planes are not fully separated because, when the string vibrates in one of the principal planes, the magnetic flux is modulated at the string location where the principal planes intersect, and consequently currents are induced in both coils. Due to the mutual interaction between magnetic fields surrounding the two pole pieces, the flux density cannot change at one pole piece without also changing at the other pole piece.
With U.S. Pat. No. 5,206,449 entitled Omniplanar Pickup for Musical Instruments, Richard E. D. McClish describes a similar arrangement of magnetic sensors, to achieve omniplanar sensitivity to string vibration. According to that invention the signals from two coils are combined after a phase shift is applied to one of the signals with respect to the other. A 90-degree phase shift is suggested for omniplanar sensitivity, and the possibility of other phase angles is mentioned. It should be noted that a 180-degree phase-shifted combination of the signals would be equivalent to a subtraction, and a zero-degree phase-shifted combination would be equivalent to a summation. With magnetic transducers of prior art, however, the sensor coils are in magnetic fields that are neither directly coupled nor fully independent. The flux fields are coupled by proximity and they intersect at the string, go that both sensor coils respond to string vibration in any direction, and they respond with different levels of sensitivity. Yet, the maximally sensitive axes of the two sensor coils are not parallel. This means that when the string vibrates in or near one of these principal planes of maximum sensitivity, the difference signal cannot result in cancellation. Hence, although a phase shifted combination of signals may provide a more nearly omniplanar sensitivity pattern than each sensor alone, neither the individual coil signals, nor their sum and difference signals, nor any phase-shifted combination of these signals can represent vibration components at intersecting planes. In contrast, if vibration components in orthogonal planes were obtained, as is the case with the present invention, then, optionally, an omniplanar output could be created from these signals.
What is needed, then, is a transducer for a vibratory string that is particularly directed to reducing cross talk between strings while providing two signals for each string representing the transverse string vibration along two orthogonal axes.
The present invention relates to variable reluctance type magnetic field sensors and has particular application to polyphonic guitar pickups. More specifically, the present invention relates to a polyphonic guitar pickup that, compared to those found in prior art, generates an output with substantially more information about the state of the vibrating string.
The present invention is directed towards a transducer for sensing the vibration of a string and resolving it into two orthogonal components by adding and subtracting the signals from two separate coils. This invention senses the string vibration in an orthogonal manner. The present invention is directed towards the use of two pickup coils, each with a pole piece of like-polarity, biased horizontally in opposite directions from the other, and a third pole piece of opposite polarity. Both coils are sensitive to transverse vibrations of strings in two orthogonal axes in the transverse plane.
The present system subtracts the signal of the first coil from the signal of the second coil to create a combined signal representing the transverse string vibrations in a first plane, and adds the signals of the first and the second coils to create a combined signal representing the transverse string vibrations in a second plane that is perpendicular to the first plane. A signal representing the mean position of the string in the first plane is also provided.
Another objective of the invention is a transducer that is sensitive to vibrations of the string above it, and substantially less sensitive to vibrations of adjacent strings.
In one preferred embodiment of the present invention, a transducer is provided with three sensor pole pieces and two electrical coils associated with a string. Two asymmetric pole pieces with sensor coils around them are located below the string and separated from one another along the axis of the string, and a symmetric pole piece is placed between them. The asymmetric pole pieces are designed to focus magnetic flux towards horizontally opposite sides of the string. When the string vibrates above all three sensor pole pieces, the motion of the string vibration along the horizontal axis will create currents of opposite polarity in the two coils. As the flux increases in the first coil to create a positive signal, the flux decreases in the second coil to create a negative signal. In contrast to this, the motion of the string vibration along the vertical axis will create currents of same polarity in the two coils. When the string vibrates along the vertical axis, as the flux increases in the first coil, the flux will also increase in the second coil, and vice versa for the decreasing flux. Therefore, when the signals from the two coils are added together, the resulting signal represents the vertical component of the string velocity, and the signals associated with the vibrations along the horizontal axis will cancel out each other. By inverting one of the signals, the two signals may be combined to form a subtraction of the signals. By subtracting the signals from the two coils, signals induced by string vibrations in the vertical plane will cancel each other out and the remaining signal will represent the vibrations in the horizontal axis. Thus, two separate audio channels will be provided where the first audio channel corresponds to the horizontal components of the string vibration and the second audio channel corresponds to the vertical components of the string vibration.
A second embodiment for the present application is the use of a magnetic saddle bridge for supporting the guitar string. By constructing the saddle bridge from a magnetically permeable material and utilizing this as a magnetic pole piece, the guitar strings will pass within the zone of the magnetic flux and engage the magnetic pickup saddle to cause the lines of magnetic flux to be carried in large part by the guitar string. This requires less magnetic energy from the permanent magnet, which will in turn reduce the cross talk between the magnetic pickup for a first string and the adjacent magnetic pickup elements for adjacent strings.
A still further embodiment of the present invention will combine the multiple sensor pole pieces and the magnetic saddle to create two signals for each string on an instrument. Thus, a hexophonic guitar pickup can utilize six separate dual coil elements for a six-string guitar and generate twelve separate guitar string signals in two sets. The first set of signals represents the vertical vibration of each of the six strings and the second set of signals represents the horizontal vibrations of each of the six strings.
A further refinement to the pickup of the present invention utilizes sensor pole caps to increase the sensitivity of the pickup by placing the sensor pole windings as perpendicular as possible to the flux lines. This allows for the coil to be placed in an area of high flux density with a large impact of the string position on the total flux across the coil.
The invention utilizes a three pole magnetic pickup for detecting string vibrations. This embodiment includes a first, symmetrically shaped magnetic pole piece with a first polarity and second and third asymmetrically shaped magnetic pole pieces where the second and third asymmetric pole pieces have a magnetic polarity opposite that of the first, symmetric pole piece. The first and second pole pieces form a first magnetic flux zone and a second magnetic flux zone extends between the first pole piece and the third pole piece. As the string vibrates, the rate of change in these magnetic flux zones is monitored through the use of electrical coils that are operatively positioned with the second and third pole pieces. The object or string is positioned so that movement of the object results in a corresponding change in the magnetic flux that is intercepted by the coils, and thereby induces a current in the coils.
These embodiments will be further described in the following detailed description.